Method of making electrically conductive contacts on substrates

ABSTRACT

A method of making an electrically conductive contact on a substrate by applying a layer of solder paste to a circuitized feature on a substrate and selectively heating and melting the solder paste over the feature to form a solder bump. The excess solder paste is removed. A focused energy heat source such as a laser beam or focused Infrared heats the solder paste. A reflective mask with apertures may be used to allow focused heating source to selectively melt areas of the solder paste layer applied to a circuitized feature. The mask and excess solder paste are removed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 09/605,617, filed on Jun. 28, 2000 now abandoned, which is adivisional of U.S. patent application Ser. No. 09/339,924; filed on Jun.24, 1999, now U.S. Pat. No. 6,173,887.

TECHNICAL FIELD

This invention relates to the formation of an electrically conductivecontact on a substrate (e.g. printed circuit board, semiconductor chipand chip carrier).

BACKGROUND OF THE INVENTION

Processes for integration of semiconductor chips and other componentsonto circuitized carriers rely on a bonding method to join theelectrodes of the chip or component to a substrate. This can be achievedby metallurgical bonding such as soldering, or by use of a conductiveadhesive. Various methods can be used to form the contacts on chips,electrical components, and on substrates. In one example a solder ballpreform of a high melt solder is fabricated first and then bonded to acomponent or substrate with a lower melt alloy. High melt solder bumpshave also been formed by evaporation techniques such as on semiconductorchips. Component bump formation and solder ball bonding are usuallyprocessed en masse, in an evaporator or oven to cause melting. Formationof high temperature bumps on organic substrates can become quiteproblematic because of the potential damage that can be caused bysubjecting the substrate or component to the elevated temperaturerequired to form the bump. The equipment and processing associated withthese technologies are relatively expensive and difficult to control forreliable bump or joint formation. More specific examples of variousmethods for forming solder-type contacts are discussed in the followingU.S. Patents and publication.

U.S. Pat. No. 5,586,715 by Schwiebert et al., a method is described toproduce solder balls by contained paste deposition using an attachedmask on non-wettable substrate, and heating the substrate-mask assemblyat a temperature to reflow the solder paste into a solder ball. Theinvention is limited in that it relies on having to use dewettablecomponents to form a ball and heating the entire substrate, mask, andsolder material en mass.

In U.S. Pat. No. 5,539,153 by Schwiebert et at., a method is describedfor bumping substrates by contained paste deposition using an attachedmask on substrate. The invention is limited in that the entiresubstrate, mask, and solder paste are all heated to the solder reflowtemperature to form the resultant solder bump and is dependent on havinga non-wettable mask to contain the solder paste.

In U.S. Pat. No. 5,658,827 by Aulicino et al., a method is described forforming solder ball contacts on substrates by squeegeeing solder pastethrough apertures in reusable mask. The method describes heating thereusable mask containing the solder and the substrate to reflowtemperature in order to form the resultant balls.

U.S. Pat. No. 4,229,232 by Kirkpatrick, describes a method for thermalprocessing on or near the surface of a metallic or dielectric materialusing a pulsed beam. The method is limited in that it thermally treatsonly the selected area's surface or near surface, rather than heatingand melting the entire solder paste layer thickness and additionallyheating the circuitized feature to form a solder bump bonded to thecircuited feature.

U.S. Pat. No. 4,832,982 by Mori et al., describes a process for forminga dispersion alloy layer on a metallic base starting with a powder alloywhich separates into two separate liquid phases when irradiated by alaser and is then quenched to form a solid on a substrate surface. Theprocess requires the formation of two liquid phases over a substratesurface rather than forming a solder bump on a circuitized feature.

U.S. Pat. No. 5,509,597 by Laferriere, describes a process and apparatuswherein solder paste is applied over a first material, a second materialis placed over the solder paste and the structure is heated by a laserbeam to produce a solder joint between two materials. The apparatus andmethod are limited in that these are concerned with soldering twomaterials together rather than forming solder bumps on circuitizedfeatures for subsequent soldering.

U.S. Pat. No. 5,272,307 by Jones, similarly describes a method andapparatus to solder leads together with heat from a laser. A work plateis used to hold the two leads together while the solder is meltedbetween the two leads. This patent does not describe forming solderbumps on circuitized features.

U.S. Pat. No. 5,156,697 by Bourell et al, describes a method andapparatus to direct a laser onto a powder comprised of a low temperaturematerial and a high temperature material to produce a bulk sinteredmass. Here the lower temperature material melts while the highertemperature remains unmelted while forming the sintered mass. Thispatent relies on two materials of differing melting temperatures andattempts to form bulk material compounds and not solder bumps on acircuitized feature.

U.S. Pat. No. 5,641,113 by Somaki et al., describes a process where asolder ball is reflowed over an electrode on a substrate to form asolder bump. Then, another ball of similar metallurgy is placed on topof the solder bump and reflowed to form an interconnection joint. Theprocess requires a stencil to contain the first reflowed solder ball andrelies on the same ball metallurgy for joining the second ball to thefirst ball.

U.S. Pat. No. 4,865,245 by Schulte et al., describes a method forjoining two semiconductor devices by applying pressure to two devices tocause a cold weld junction between respective solder bumps of eachsemiconductor. The method relies strictly on the ability to cold weldbetween two electrode bumps and requires applying potentially damagingpressure to the semiconductor devices which could cause crackinitiation.

U.S. Pat. No. 3,836,745 by Costello, describes a method of soldercoating and joining members by heating a predetermined portion ofmembers through the use of radiant energy adsorbent material depositedon the members. The method is limited in that it requires the heating ofthe entire ribbon layer of solder paste between members and overmembers, relying on wicking away of the molten solder between themembers to prevent solder bridging.

IBM Technical Disclosure Bulletin, Vol. 38, No. 05, May 1995, describesan additive that is added to solder paste to improve the adsorptioncharacteristic of the solder paste to reduce the energy required of alaser to cause solder reflow.

This publication does not describe formation of solder bumps oncircuitized features using laser or focused infrared energy source.

It is believed that the method of selective heating of a layer of solderpaste over a circuitized feature having the advantageous features citedherein and otherwise discernible from the aforementioned teachings wouldrepresent a significant advancement in the art.

DISCLOSURE OF THE INVENTION

It is a primary object of this invention to enhance the art of providingelectrical contacts on circuitized substrates.

It is a further object to provide a method of making such contactswherein solder forms an important part thereof.

It is yet another object of the invention to provide a method forselectively heating an area of solder paste over a circuitized featureto form a solder bump which can be used for a subsequent interconnectionto contacts of another component or components.

According to one aspect of the invention, there is provided a method ofmaking at least one electrically conductive contact on a substratecomprising the steps of: providing a substrate with a surface having atleast one circuitized feature; applying a layer of solder paste over thesurface of the substrate and at least a portion of the circuitizedfeature; providing a mask having a top surface and a bottom surface andat least one opening; orienting the mask such that at least one openingin the mask is aligned with the circuitized feature; positioning thebottom surface of the mask on the layer of the solder paste; directing afocussed radiation beam having a beam diameter larger than said maskopening on said solder paste through said opening in said mask andselectively heating substantially only the solder paste over thecircuitized feature through the mask sufficiently to melt the solderpaste over the circuitized feature to form a substantially solidconductive solder bump while not significantly heating the remainder ofthe solder paste over the surface of the substrate or substantiallyaltering the configuration; removing the mask and removing the solderpaste that was not significantly heated.

Another aspect of the invention is a method of making at least oneelectrically conductive contact on a substrate comprising the steps of:providing a substrate with a surface having at least one circuitizedfeature; applying a layer of solder paste over the surface of thesubstrate and at least a portion of the circuitized feature; providing amask having a top surface and a bottom surface and at least one opening;orienting the mask such that at least one opening in the mask is alignedwith the circuitized feature; positioning the mask immediately above thelayer of solder paste at a spaced distance; directing a focussedradiation beam having a beam diameter larger than said mask opening onsaid solder paste through said opening in said mask and selectivelyheating substantially only the solder paste over the circuitized featurethrough the mask sufficiently to melt the solder paste over thecircuitized feature to form a substantially solid conductive solder bumpwhile not significantly heating the remainder of the solder paste orsubstantially altering the configuration, the heating and forming of theconductive solder bump occurring without the mask contacting the solderpaste or substrate; removing the mask; and thereafter removing thesolder paste that was not significantly heated.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention are described with reference tothe accompanying diagrammatic section drawings, in which:

FIGS. 1-3 illustrate one embodiment of the invention showing steps toform solder bumps on circuitized features from a layer of solder pasteapplied over a circuitized substrate, and providing selective heatingthereof;

FIGS. 4-7 illustrate another embodiment of the invention showing stepsto form solder bumps on circuitized features from a layer of solderpaste applied over a circuitized substrate and providing selectiveheating thereof through apertures in a mask;

FIGS. 8-10 illustrate yet another embodiment of the invention showingsteps to form solder bumps on circuitized features using, again, a maskwhich has apertures filled with solder paste, providing selectiveheating thereof, and removing the mask;

FIG. 11 illustrates an embodiment showing the formation of solder bumpson two representative circuitized features on a substrate from a layerof solder paste which substantially covers the circuitized features, andproviding selective heating thereof; and

FIG. 12 illustrates an embodiment showing the formation of solder bumpson two representative circuitized features on a substrate, from a layerof solder paste which in one example substantially covers a circuitizedfeature and in another, partially covers a circuitized feature, andproviding selective heating thereof.

BEST MODE FOR CARRYING OUT THE INVENTION

For a better understanding of the present invention, together with otherand further objects, advantages and capabilities thereof, reference ismade to the following disclosure and appended claims in connection withthe above- described drawings.

Referring to FIG. 1, a substrate or electrical component 101 (e.g., aprinted circuit board) includes circuitized features 102 formed byelectrolytic or electroless plating techniques or by sputtering.Typically, these circuitized features are plated copper or otherelectrically conductive metal and are typically 0.001″ thick. A solderpaste layer 103, with a thickness of about 0.002″ to 0.010″, is appliedover the surface of the substrate and over the circuitized features. Awater soluble solder paste which melts at 183 deg. C., such as 63 wt. %lead and 37 wt. % tin, may be used to form eutectic solder bumps. Awater soluble solder paste which melts at 302 deg. C., such as 90 wt. %lead and 10 wt. % tin, may also be used to form high temperature solderbumps. One example is Indalloy #159, manufactured by Indium Corp.,Utica, N.Y. The layer of solder covers the complete surface of thesubstrate and circuitized features in FIG. 1. A layer thickness of0.003″ is preferably used, but this will depend on how much of thefeature needs to be covered with a solder bump and the size or height ofthe bump desired. The paste layer need not cover the complete surface ofthe substrate but can cover a partial area of the substrate surface andits associated circuitized feature, or just a partial area of acircuitized feature. Examples are shown in FIG. 11. The solder paste 103can be dried on the substrate in an oven at 110 degrees F. for 60minutes or air dried, to remove the paste solvents and any solvents usedto facilitate handling.

Referring to FIG. 2, a beam from a laser or infrared energy source 104melts the solder paste over the circuitized feature 102 to form a solderbump or ball 105. The solder paste 103 which was not treated by theenergy source 104 remains as unmelted solder paste. A Nd:YAG laser, 1064nanometers wavelength, continuous wave laser, model 570M, manufacturedby Lee Laser, Inc., Orlando, Fla., may be used for selective soldermelting of low temperature solder pastes. A pulse duration of 200microseconds to one second, at 3 watts output, will successfully form asolder bump on a 0.003″ thick layer of solder paste over a 0.001″ thickcopper contact pad. The resulting bump will have a width substantiallythe same as the pad width, and a height of from about 0.001″ to about0.050″. Note that this bump configuration is semi-spherical, with arounded upper curved surface 130.

For higher temperature applications, such as with high melt solderpastes, a 250 Hz CO2 Laser, model Diamond 64, manufactured by Coherent,Inc., Laser Products Division, Santa Clara, Calif., can be used. Bumpshave been formed using 20 to 50 pulses per solder bump, each pulse beingapproximately 100 microseconds at 3 watts, on a 0.003″ thick solderpaste layer over a 0.001″ copper contact pad.

Infrared energy can be generated from a linear source such as ResearchInc., Model 5215 (available from Research Energy Systems Div.,Minneapolis, Minn.), with an energy output of 400 watts/linear inch, anda line width of 00.18″, using a parabolic reflective mirror. A spotinfrared source, such as Research Inc., Model 4085, with an energy of650 watts/in² can also be used. Treatment times of ½ to 120 seconds canbe used.

While specific laser and infrared parameters have been recited here, itis recognized by those skilled in the art that other parameter settingsmay be desirable, depending on work piece, solder paste thickness,solder paste composition, and pad thickness for example.

Referring to FIG. 3, after the solder bumps 105 have been allowed tocool and solidify, the unmelted residual solder paste is removed,preferably with a distilled water wash. This leaves the substrate 101free of solder paste between the circuitized features 102, each now witha solder bump 105 thereon.

FIGS. 4 through 7 illustrate another embodiment of the invention whereina reflective mask with apertures is used to selectively allow an energysource (such as a scanning laser or focused infrared) to melt theexposed solder paste while protecting those areas where melting is notdesired. The mask can be made from a material with low coefficient ofthermal expansion, such as molybdenum with a gold flash plating (toimprove reflectivity). Lowering the temperature of the mask andsubstrate surface to a minimum is strongly desired, particularly whilethe selective melting is occurring. The molybdenum and reflectivecoating assure this feature. The mask may be placed on the solder pastein contact therewith or may positioned so that it is over the solderpaste but not in contact with it.

FIG. 4 shows a substrate 101, with circuitized features 102, and coveredwith a layer of solder paste 103.

FIG. 5 shows a mask 106 with apertures 107. The mask is oriented withrespect to the so that at least one opening is aligned with at least onecircuitized feature, and preferably so that a number of openings are soaligned. The top surface 106A of the mask is, preferably, polished orcoated with a reflective material such as a gold film. An energy source,preferably a laser, even though such source may also be any of theexamples described earlier, generates a radiation beam 104 which ispreferably a focussed radiation beam. The radiation beam has a diameter101 at the point of impact on the mask 106. This diameter is larger thanthe aperture 107 so that only a portion of the incident beam 104 passesthrough to the Solder paste 103, and melts the solder paste 103 over theselected areas of the circuitized features 102. Solder paste 103A whichis shielded by the mask material does not reach melting temperature andremains as paste.

FIG. 6 shows the substrate 101 after the mask is removed. A solid solderbump 105 is formed, upon cooling, on the circuitized feature 102. Theenergy source heats the paste down to the surface of the circuitizedfeature 102, sufficient to cause the solder paste to melt andmetallurgically bond to the circuitized feature. Unmelted solder paste103A remains as paste. Bumps 105 included the rounded upper surface 130.

FIG. 7 shows the finished substrate 101 wherein the unmelted solderpaste 103A has been removed, such as with a water wash, leaving thecircuitized features 102 with solder bumps 105. These bumps are similarin size to the bumps formed in FIGS. 1-3.

FIGS. 8-10, show an embodiment of the invention wherein a substrate 101with circuitized features 102 has a mask 108 or film with apertures 109pre-filled with solder paste 103, placed on the upper surface 112 of thesubstrate, the pre-filled apertures each aligning with respective onesof the circuitized features. Selective heating by laser or infraredmelts the solder paste, forming a solder bump on the circuitizedfeature.

More particularly, FIG. 8 shows a cross-sectional view of such a film ormask 108 with a bottom surface 110. An optional layer of adhesive 125may be applied to bottom surface 110. Apertures 109 are filled withsolder paste 103, using a squeegee or a dispensing nozzle. Drying thesolder paste while in the film tape facilitates handling of the filmtape. The film 108 can be fabricated from polyimide made by E. I DuPontde Nemours & Co., Inc., Wilmington, Del., such as KAPTON (TM) or KAPTONtape with adhesive. Apertures 111 may be formed by mechanical means,laser drilling, or by chemical etching with a solution, e.g., potassiumhydroxide and/or potassium carbonate. An optional layer of mild siliconeadhesive 125 made by Furon, New Haven, Conn., product number K250 orK2554, can be applied to the bottom surface of the film, to form a tape,which improves contact with the substrate top surface 112.

FIG. 9 shows mask 108 placed on the top surface 112 of substrate 101.The apertures 109, which are pre-filled with solder paste, are alignedwith the circuitized features 102. Focused energy source 104 melts thesolder paste within the apertures 111 of the mask 108 while the mask ispositioned as shown in FIG. 9. Thus, the mask shields substrate 101 fromsome heat generated during this process. FIG. 10 shows the substratewith the mask and adhesive layer removed after the solder bmumps 105have been formed on the circuitized features.

FIG. 10 shows the substrate 101 with the mask 108 and adhesive layer 125removed after the solder bumps 105 have been formed on the circuitizedfeatures 102. Examples of some energy sources with their recommendedparameter settings were discussed earlier.

Continuing with FIG. 11, a substrate 101 has circuitized features 122and 123 which can be coated with solder paste shown by the areacontained within the peripheral line 113. A focused energy sourceselectively melts the solder paste as shown by 118, 119, 120 and 121.

Alternatively, as shown in FIG. 12, previously described circuitizedfeatures 122 and 123 on substrate 101, can be covered with a layer ofpaste, represented by 114, 115, 116 and 117. These smaller areas ofsolder paste can be applied over the complete circuit feature 123, orpartially over a circuit feature 122. As previously described above, afocused energy source selectively melts the solder paste as shown by118, 119, 120 and 121.

The invention uses focused energy sources to rapidly melt solder pastein selected areas over desired circuitized features, without damagingordinarily heat sensitive materials (e.g., those of the underlyingsubstrate). High temperature solder bumps may be formed on heatintolerant substrates without causing damage. While the ability to formhigh temperature solder bumps is a distinct advantage, those withordinary skill in the art will recognize that the invention can uselower melt temperature pastes and alloys to form the solder coatings orformations.

Interconnect structures can be formed between first and secondsubstrates each having high temperature bumps made by the focused energymelting of solder paste described herein. In this example, a lowertemperature metal or alloy can be applied to the bumps on the firstsubstrate. The bumps on the second substrate can then be flattened andfluxed, the two substrates brought together to contact the respectivebump arrays thereof, and heat applied sufficient to melt the lowtemperature metal, thus forming several metallurgical bonds.

While there have been shown and described what are at present consideredthe preferred embodiments of the invention, it will be obvious to thoseskilled in the art that various modifications made be made thereinwithout departing from the scope of the invention as defined by theappended claims.

What is claimed:
 1. A method of making at least one electricallyconductive contact on a substrate, said method comprising the steps of:providing a substrate with a surface having at least one circuitizedfeature thereon; applying a layer of solder paste over said surface ofsaid substrate and at least a portion of said at least one circuitizedfeature; providing a mask, having a top surface and a bottom surface,and at least one opening therein; orienting said mask such that said atleast one opening in said mask is aligned with said at least onecircuitized feature; positioning said bottom surface of said mask onsaid layer of said solder paste; directing a radiation beam having abeam diameter larger than said mask opening on said solder paste throughsaid opening in said mask, and selectively heating substantially onlysaid solder paste over said at least one circuitized feature throughsaid mask under said opening sufficiently to melt said solder paste oversaid at least one circuitized feature to form a substantially solidconductive solder bump commensurate with said mask opening on said atleast one circuitized feature while not significantly heating theremainder of said solder paste over said surface of said substrate orsubstantially altering the configuration thereof; removing said mask;and removing said solder paste that was not significantly heated.
 2. Themethod of claim 1 further including the step of substantially dryingsaid layer of solder paste before said selectively heating.
 3. Themethod of claim 1 wherein said selectively heating of said solder pasteis achieved using a laser light source.
 4. The method of claim 3 furtherincluding the step of polishing said top surface of said mask.
 5. Themethod of claim 3 further including the step of providing said topsurface of said mask with a layer of reflective material.
 6. The methodof claim 1 wherein said selectively heating of said solder paste isachieved by using infrared energy.
 7. The method of claim 6 furtherincluding the step of polishing said top surface of said mask.
 8. Themethod of claim 6 further including the step of providing said topsurface of said mask with a layer of reflective material.
 9. The methodaccording to claim 1 wherein said energy beam is a focussed energy beam.10. The method according to claim 9 wherein said focussed energy beam isa laser beam.
 11. A method of making at least one electricallyconductive contact on a substrate, said method comprising the steps of:providing a substrate with a surface having at least one circuitizedfeature thereon; applying a layer of solder paste over said surface ofsaid substrate and at least a portion of said at least one circuitizedfeature; providing a mask, having a top surface and a bottom surface,and at least one opening therein; orienting said mask such that said atleast one opening in said mask is aligned with said at least onecircuitized feature; positioning said mask immediately above said layerof said solder paste at a spaced distance therefrom; directing aradiation beam having a beam diameter larger than said mask opening onsaid solder paste through said opening in said mask, and selectivelyheating substantially only said solder paste over said at least onecircuitized feature through said mask under said opening sufficiently tomelt said solder paste over said at least one circuitized feature toform a substantially solid conductive solder bump on said circuitizedfeature commensurate with said mask opening, while not significantlyheating the remainder of said solder paste or substantially altering theconfiguration thereof, said heating and forming of said conductivesolder bump occurring while said mask is positioned at said spaceddistance from said solder paste; removing said mask; and removing saidsolder paste that was not significantly heated.
 12. The method of claim11 further including the step of substantially drying said layer ofsolder paste before said selectively heating.
 13. The method of claim 11wherein said heating of said solder paste is achieved using a laserlight source.
 14. The method of claim 11 further including the step ofpolishing said top surface of said mask.
 15. The method of claim 14further including the step of providing said top surface of said maskwith a layer of reflective material.
 16. The method of claim 11 whereinsaid heating of said solder paste is achieved by using infrared energy.17. The method of claim 16 further including the step of polishing saidtop surface of said mask.
 18. The method of claim 16 further includingthe step of providing said top surface of said mask with a layer ofreflective material.
 19. The method according to claim 11 wherein saidenergy beam is a focussed energy beam.
 20. The method according to claim19 wherein said focussed energy beam is a laser beam.